What is the difference among Alpha Over / Alpha Under / Over Drop?
To my surprise, my following report here confirmed that there was something wrong here since 2.4x. Thanks to psy-fi (Antony Riakiotakis), the developer who has helped to fix it in commits here and here.

Now it makes sense that, as Wiki said, Over Drop is actually a "mixture" of Alpha Over and Alpha Under. In other words, the first selected strip would be the foreground (same as Alpha Under), while the fade value also controls this foreground (same as Alpha Over).

For easy understanding: